Use symbolic names instead of magic values for the position parameter of get_irn_n().

[r21364]

ir/be/ia32/bearch_ia32.c

@@ -630,9 +630,10 @@ static int ia32_get_op_estimated_cost(const ir_node *irn)
 			(we assume they are in cache), other memory operations cost 20
 			cycles.
 		*/
-		if(is_ia32_use_frame(irn) ||
-				(is_ia32_NoReg_GP(get_irn_n(irn, 0)) &&
-		         is_ia32_NoReg_GP(get_irn_n(irn, 1)))) {
+		if (is_ia32_use_frame(irn) || (
+		    	is_ia32_NoReg_GP(get_irn_n(irn, n_ia32_base)) &&
+		    	is_ia32_NoReg_GP(get_irn_n(irn, n_ia32_index))
+		    )) {
 			cost += 5;
 		} else {
 			cost += 20;